Band pass coupling system



Aug. 18, 1942. R. DE COLA BAND PASS COUPLING SYSTEM 2 Sheets-Sheet 1Filed April 22, 1940 ATITENTUATION m DECIBELS PHASE ANGLE-- Aug. 18,194-2. 5 2,293,384

BAND PASS COUPLING SYSTEM Filed April 22, 1940 2 Sheets-Shet? ceiver.

a frequency of 8.25 megacycles.

Patented Aug. 18, 1942 BANl) PASSCOUPLING SYSTEM Rinaldo De Cola,Chicago, IlL, assignor to Belmont RadioCorporation, Chicago, 111., acor-' poration of Illinois f Application April 22, 1940, Serial No.330,881

16 Claims.

The present invention relates in general to band pass coupling systems,and more in particular to coupling systems of this character which aresuitable for use at the intermediate frequency stages in the videochannel of a television re- Specifically, the invention is a new andimproved intermediate frequency transformer adapted to pass thecomparatively wide band of frequencies used for video signals and havinggreat attenuation for an adjacent frequency In order to show the actualrequirements, it may be stated that the general practice is to use anintermediate frequency band extending from about 8.75 megacycles to12.75 megacycles for the video signals, which brings the audio signalsfor the television channel undergoing reception in at The spacing of thetelevision channels is such that the audio signals for the next adjacenttelevision channelappear at an intermediate frequency of, 14.25megacycles. The intermediate frequency stages in the video channel ofthe receiver must therefore pro...

vide for uniform transmission over the range 8.75

" to 12.75 megacycles, and at the same time must provide adequateattenuation for the sound signals at frequencies of 8.25 and 14.25megacycles.

The problem of producing the desired attenuationfor frequencies adjacentto the picture signal frequency band has received some attentionfrequency transformer which will give normal transmission of the picturefrequency band, and at the same time produce relatively great attenueation of the sound channel. Moreover, attenuation is provided over afairly good range of frequencies including the sound channel frequency,so that there is no danger that the latter will shift outside the rangeof attenuation during reception, as might otherwise be the case due to'band on which audio signals are transmitted.

slight changes in the receiver oscillator frequency.

The frequency of maximum attenuation may be lower or higher than thefrequency of the picture'signal transmission band. In the case of atransformer designed to produce attenuation at a lower frequency, thefrequency of maximum attenuation is made to coincide with the frequencyof the sound carrier which is associated with the television channelundergoing reception, while if the transformer is designed to provideattenuation at a higher frequency, the attenuation frequency will be thesame as the frequency of the 7 sound carrier of the next adjacenttelevision channel. The transformers at different stages may thereforebe of two types, designed to atten uate frequencies below and above thepicture signal band, respectively, whereby both adjacent sound carriersmay be eliminated.

heretofore, but the solutions proposed have not been entirelysatisfactory. One'type of proposed solution utilizes so-called trapcircuits-which act as short circuits for the frequency to be attenuated,but such systems fail to produce the requisite attenuation due to theradio frequency resistance of the trap elements. Other arrangementswhich have been proposed are objectionable because of the extremelynarrow band of frequencies over which attenuation is obtained.

The present invention affords a solution which is substantially free ofthe objectionable features of prior systems. It provides a couplingsystem for use at an intermediate frequency stage, that is, anintermediate frequency transformer, having two different circuit pathsbetween the input and output circuits, and having the constants of thesystem so adjusted thatat the frequency at Further features of theinvention will be ex plained in the course of the ensuingdetaileddescription, and with reference to'the accompanying drawings, inwhich:

Fig. 1 is a diagrammatic circuit drawing of an intermediate frequencytransformer according to the invention, as used at one of theintermediate frequency stages of a television receiver;

Fig. 2 is an equivalent theoretical circuitdiagram of the transformershown in Fig. 1, which is useful in discussing the operation thereof;

Fig. 3 is a diagrammatic circuit drawing showing a modification, andFig. 4 is the equivalent .theoretical circuit diagram;

Fig. 5 is a diagrammatic circuit drawing showing a-further modification,Fig. 6 being the equivwhich attenuation is desired the currents in thetwo-circuit paths induce equal opposing voltages in -the output circuitand produce cancellation.

At the same time the necessary flat response characteristic over thefrequency band which is to be transmitted is maintained.

The invention thus provides an intermediate alent theoretical circuitdiagram; I

Fig. '7 shows the physical arrangement of the transformer windings inthatform of the invention which is shown in Fig. 1; v v Figs. 8 and 9show winding arrangements that may be used with the modification of Fig.3, the arrangement of Fig. 9, with a different coil spacing, being alsousable" with the modification of Fig. 5;

Figs. 10 to 13, inclusive, are graphs which show selectivity curves sucha may be obtained with variable resistor l5.

transformers constructed in accordance with the invention; and

Fig. 14 is a graph which shows typical phase angle curves over'a rangeincluding the transmission band and attenuation frequencies.

Referring to Fig. l, the reference character 2 indicates a vacuum tube,preferably of the pentode type, which may be an amplifier tube at anintermediate frequency stage of a television receiver. The vacuum tube 2is provided with input terminals 3 and t, at which connections are madefrom the preceding stage. A resistor 5 may be included in the cathodecircuit to provide the proper bias for the control grid. The connectionsfor the screen and suppressor grids are not shown, as they are of theusual character.

The plate circuit of tube 2 includes the battery 9, representing -anysuitable source of plus B potential, and also includes a tunedcircuit/2i comprising the inductance H and the variable condenser l0.This tuned circuit may be regarded as the input circuit of thetransformer.

Adjacent to the tuned circuit 28 there is shown a second tuned circuit22, comprising the inductance 93, the variable condenser Hi, and theThis circuit is preferably grounded as shown.

A third tuned circuit 23 is also shown, which comprises the inductancel6 and variable condenser l8. The circuit 23 may be regarded as theoutput circuit of the transformer and may be connected to the grid ofthe pentode 20, which may be similar to the tube 2. A suitable loadresistor I9 is used, shunting the condenser iii. The complete gridcircuit for tube 20 includes the automatic volume control lead, asindicated.

The transformer also includes a fourth tuned circuit 24 comprising theinductance l2, inductance I I, and variable condenser 25. This circuitmay be grounded as shown in the drawings.

may be small trimmer condensers of the mica compression type, having arange of about 5 to 15 mmf. The load resistor l9 may have a value ofabout 2000 ohms. The value of the resistor I5 is indicated as variable,but a fixed resistor may be used, after the correct value has beenascertained.

The inductances ll, I2, l3, l6, and I7 are the windings of thetransformer and are shown in their proper physical relation in Fig. '7.The socircuit 24. In these circumstances it will be clear that voltageswill be induced in the winding l6 due to the currents in circuits 22 and24, and that the effective voltage'will be equal to the vector sum ofthese voltages. The operation of the transformer depends on the factthat the induced voltages are essentially aiding over the picturefrequency band, but at the frequency of the sound, carrier, where greatattenuation is desired, these induced voltages are equal and opposing,thereby reducing the effective voltage and the current in the outputcircuit 23 substantially to zero.

The manner in which the desired result is accomplished can be explainedmore conveniently with reference to the theoretical circuit Fig. 2. Inthis drawing the several tuned circuits are indicated by the samereference numerals as in Fig. l, and can readily be identified. Themutual couplings between the circuits and other constants are indicatedby conventional symbols.

In general, it may be stated that the selective action of thetransformer depends on the proper arrangement of the polarities of themutual coupling reactances, upon the relative phase shift betweencurrents I2 and I4 which occurs over the range which includes the bandof frequencies which are to be transmitted and the frequency to beattenuated, and upon changes in the relative amplitudes of the currentswhich take place over this frequency range.

.40 The condensers shown in the tuned circuits called universalwinding'is preferably used but is t not absolutely essential. The coilsare mounted on a small Bakelite tube 26 and are slidablerelative to eachother so that the coupling can be adjusted. From the positions of thecoils as shown in Fig. 7, it will be seen that coill l is inductivelycoupled to coils l2 and 93, while coil I6 is inductively coupled tocoils I3 and IT. This relation is also indicated in Fig. 1, where theinput circuit 2| of the transformer is shown as inductively coupled tocircuits 22 and 2d, while the two latter circuits are shown asinductively coupled to the output circuit 23.

From the foregoing it will be appreciated that there are two circuitpaths over which energy may be transmitted from the plate circuit oftube 2 to the grid circuit of tube 20. One of these paths includes thetuned circuits 2|, 22, and 23, while the other path includes the tunedcircuits 2| 24, and 23. Stating it another way, the input circuit 2| ofthe transformer is coupled to the output circuit 23 by way of twointermediate or link .circuits, one of which is the tuned circuit22'while the other is the tuned The relation between these factors andthe manner in which they affect the operation of the system can perhapsbe best explained and understood with the aid of a brief mathematicalanalysis of the theoretical circuit, Fig. 2, which will now be proceededwith. Taking into account the four coupling reactances involved, andassuming that coupling M4 is negative, the equation for the current I:in the input circuit 23 can be written in the form where D and D' arethe determinants of the system. Maximum attenuation will be obtainedwhen 11 becomes equal to zero, or when I3=O=D=M3M4Z2M1M2Z4 (2) where M1,M2, M3 and M4 are the mutual coupling reactances, and Z2 and Z4 are theimpedances of circuits 22 and 24. written with good accuracy asZ2=R2(1!,7'262Q2) and similarly Z4 can be written as Z4=R4(1+7'264Q4)where 62 is equal to Now Z2 can be and 64 is equal to the frequencies isand f4 being ,the resonant fre- 0: (K3K4L4R2K 1K 2L2R4)y'zLzLdKsKqfiz-KrKzfiQ (5) The right hand portion of Equation 5compositive, with the exception of M4.

other combinations, which will reduce current I:

I prises .two expressions which must each equal zero if the right handportion ofthe equation is equal to zero. The first expression reduces tozero when i mm c. (6)

the symbol Q2 being written for the expression 21rfL2/R2, and Q4 beingwritten for .the expression 21rfL4/R4. hand portion of Equation 5reduces to zero when (resistance balance) m (reactance balance) (7) Asimultaneous balance is obtained, that is, Equation 5 is fullysatisfied, when are From Equation 8 the equation for the frequency ofinfinite attenuation can readily be derived and may be written in theform of In the foregoing discussion of Fig. 2, all the mutual couplingreactances were assumed to be Various to zero at a particular frequencyare possible, as may be perceived from inspection of Equation 2. Allthese combinations satisfy the requirement that for any givencombination of couplings in the two circuit paths all the couplings,except one, have the same polarity. Unless this require- The' secondexpressionin the right fl and is indicated at f. Curves 2 and a4 showthe manner in which the corresponding currents I: and I4 lead or lag theimpressed voltages at the different. frequencies. At the resonancefrequency the current I: is in phase with the voltage,

as indicated by the fact that curve t: crosses f:

on the zero line. At frequencies higher than In current I: has a laggingcharacteristic, while at lower frequencies it leads the voltage. CurrentI4 behaves in a generally similar manner, as indicated by curve 4,although the phase angle changes more rapidly, due to its lowerdissipation or resistance. At frequency f currents I: and I4 are bothleading at the same angle and are in phase with each other due to thefact that the voltages are ,in phase (couplings M1 and Mzbeing bothpositive), which gives the correct phase relation for cancellation withthe negative coupling M4.

As regards the current magnitudes, it will be seen also that atfrequency f, currents I: and I4 M4 are equal.

ment is met, cancellation in the output circuit 23 cannot result; thatis, current I3 cannot be reduced to zero.

It will be of advantage now to consider one to zero at some particularfrequency where currents I: and I4 are in phase, whereby opposingvoltages are induced in L3, provided that the current magnitudesand theconcerned mutual couplings are so related that the induced voltages areequal. This last condition is satisfied if I27'WM2 is equal to I4iWM4.The foregoing states generally the conditions as regards phase andmagnitude relation of currents I2 and I4 which obtain ,at the frequencyof maximum attenuation, when using the particular coupling combinationunder consideration. i

The phase relation between currents I: and I4 may be discussed more. indetail with reference trated in Fig. 14.

are equal in magnitude, which satisfies another condition forcancellation, assuming that M: and

It will be understood that currents I2 and.I do not necessarily have tobe equal at frequency 1, so long as the mutual couplings M2 and M4 aresuch as to produce equal induced voltages in circuit 23. It will benoticed that-at frequency 1 both the phase angles and magnitudes of I2and I4 are changing rather slowly, which accounts for the excellentattenuation over a fairly wide range including frequency f.

Since currents I2, and I4 are in phase at frequency f, the frequency ofmaximum attenuation,

it follows that they must be out of phase, or differ in magnitude, orboth, at frequencies within the picture transmission band, whichincludes frequencies f: and f4. This condition is also illus- Aspreviously intimated, the attenuation frequency f of the transformerdoes not necessarily correspond to the sound channel frequency of 8.25megacycles and can be shifted to a'point above the picture signalfrequency band, if desired, to' the sound carrier frequency of 14.25That such shift is megacycles, for instance. possible will be evidentfrom Equation 9, which shows that ,if frequency fl is lower than,frequency h, then frequency i will be lower than frequency f4, while iffrequency I4 is higher than frequency f2, then frequency f is higherthan frequency f4. This means that if frequency {is 1 to be shifted to apoint above the picture transto Fig. 14, which shows graphically thephase angles at which the currents Ia and I4 lead or lag thecorresponding voltages over. the frequency range with which we areconcerned.- In this figure, I2 is the resonance frequency of circuit 22(Zn) and i4 is the resonance frequency of circuit 24 (Z4), while I2 andI4 are the corresponding resonance curves. The frequency of maximumattenuation is assumed to be lower than I: and

mission band, the .relative positions of frequencies f4 and f2 as shownin Fig. 14 mustbe. reversed; that is, frequency f4 must be higher thanfrequency f2. Frequency f4 can be adjusted to the properpoint' byvarying condenser C4. It may also be necessary to relocate frequency f2,which can be done by adjusting condenser C2. The other constants,including resistance R2. will of course require readjustment. Theseadiijus ments will shortly be discussed more in deail.

It should be noted that in any case. and this is true of themodifications which will be described later on as well as of the ,oneunder discussion, the frequency of maximum attenuation, does notcorrespond to the resonance frequency of any of the tuned circuits. Thisis an important and fundamental characteristic fea-- ture of theinvention, which diffentiates it from known coupling systems, in whichthe resonance frequency of one at least of the tuned circuits alwayscoincides with the attenuation frequency.

The calculation of the response curves of current I: from Equation 1over the concerned frequency range and with the'conditions required byEquations 6, 7, and 8 is possible but is exceedingly tedious. The properadjustment of the constants of the transformer which is required inorder to satisfy the equations is p'referably determined experimentally,therefore.

.A primary consideration, of course, is uniform transmission over therequired. frequency range, which is obtained by properly adjusting allfour coupling factors and by adjusting the condensers C1, C2, and C3.These adjustments fix the limits of the frequency range at the desiredpoints, and the load resistor 89 associated with tuned circuit 23produces the necessary flat top characteristic over this range. Thephase characteristic from the input side of the transformer to theoutput side has been checked and has been found to be quite uniform overthe transmission range.

The next step is to locate the attenuation frequency ,1, which is doneby adjusting the condenser C4. The adjustment of. this condenser fixesthe location of frequency ii, which in turn determines frequency ,1,from Equation 9. In other words, the frequency f at which attenuation isto be obtained is fixed at the desired point by properly locatingfrequency f; with reference to frequency ii. The adjustment of condenserC4 therefore produces the required frequency relation to satisfyEquation 7, and its effect can be visualized by reference to Fig. 14,from which it can be seen that a shift inthe location of frequency f4will correspondingly shift the phase angle curve 4:4. Thus the phaseangle curve m is made to intersect the phase angle curve 2 at thedesired attenuation frequency f.

Having made the necessary adjustments to insure that currents I2 and I4will be in phase at the desired attenuation frequency, the next step isto bring the current magnitudes into the properrelation to satisfyEquation 6, which is done by adjusting the resistance R2. Here again theoperation can be visualized from Fig. 14, from which it can be seen thatif the curve I: were to be raised, the intersection of the currentcurves would occur at a frequency higher than frequency f andcancellation would not take place because the phase curve intersectionand the current curve intersection would no longer be at the same pointon the frequency axis. If the curve of current 12 were to be lowered thesame condition would exist, except that the current curves wouldintersect at too low a frequency. The value of resistance R2 determinesthe height of current curve I2, and by adjusting this resistance thecurve can be shifted up .or down until it intersects curve 14 at thedesired frequency f. The phase angle curves and the current curves nowintersect at the same frequency, Equation 8 is satisfied, and maximumattenuation is obtained.

The results actually obtained in practice with the transformer of Fig. lare illustrated by the selectivity curve Fig. 13, where attenuation isdecibels is plotted against frequency, In plotting this curve the pointof zero attenuation is arbitrarilyfixed' at the lowest point of thecurve 'which is within the frequency range to be transtion is obtainedat the frequency of the accompanying sound'carrier, or at a frequency of8.25 megacycles, the attenuation at this frequency being on the order ofnearly 60 decibels. While the frequency range over which the greatattenuation is obtained is narrow compared to the picture signal range,consideration of the frequency scale will show that at points not farfrom the bottom of the curve the attenuation band is many times thewidth of the sound channel. At 50 decibels, for instance, theattenuation band has a width ofat least 25Q,000 cycles, while the soundsignals cover a range of only about 10,000 cycles. I

The selectivity curve Fig. 18 also shows a good order of attenuation fora frequency of 14.25 megacycles, which is the frequency of the soundcarrier of the adjacent television channel. The excellent attenuation at14.25 megacycles is accounted for by the direct coupling between L1.

and and can be predicted mathematically if an additional coupling factordesignating the coupling between Li and L3 is introduced into Equation1.

In the foregoing explanation it was assumed that all mutual couplingsare positive except M4, and it has been explained in detail howcancellation is efiected at a particular frequency. The transformer,operates in a similar manner if the mutual coupling M2 is negative, allother couplings being positive. Coupling M2 can be made negative byreversing coils it and ll, Fig. 1, corresponding to inductances In and12/4, Fig. 2.

A somewhat difierent case is presented if coupling M is made negative byreversing coil 82, Fig. 1, corresponding to inductance L'-'4, Fig. 2,all other couplings including M4 being positive. The phase relationbetween currents I2 and I; may be essentially the same as depicted inFig. 14, except that current I4 is shifted in phase relative to currentIs by 180 degrees, due to the fact that L"4 is reversed. The phase anglecurve 4 in Fig, 14 must therefore be regarded as being shifted down to anew zero axis which is 180 degrees out of phase with the zero axisshown. With this understanding, it may be stated that at frequency fcurrents I2 and I4 lead their respective voltages by the same phaseangles, but are 180 degrees out of phase. As couplings M2 and M4 areboth assumed to be positive, the voltages induced in La oppose eachother, and cancellation results if the proper relation exists betweenthe current magnitudes and couplings as previously explained.

A case similar to the last described case is presented if coupling M1 ismade negative, which can be done by reversing coils II and I2, Fig. 1,

corresponding toinductances L1 and L4, Fig. 2.

It will be clear that in this case currents I2 and I4 will again be outof phase by degrees at frequency .f and that the operation will besimilar to the operation just described.

All these variations come under the general rule which states that inorder to effect cancellation at some desired frequency all the mutualcouplings, except one, must have the same polarity, and the mathematicaltreatment is the same in each case.

'Attention may now be directed to the modification which is shown inFig. 3. Like Fig. 1, this figure shows two vacuum tubes 32 and '43,which are assumed to be located at adjacent'intermediate frequencystages in a television receiver. The plate circuit of tube 32 isconnected to the grid circuit of tube 43 by means of a modieludesinductance 40 and condenser 4|. The inductances are the windings of thetransformer. Two arrangements of the transformer windings or coils arepossible, as illustrated in Figs. 8 and 9. 7

According to Fig. 8, windings 38 and 39 are combined and form a singlecoil which is located between the windings or coils 31 and d0. With thisarrangement it will be seen that circuit 5| is inductively coupled tocircuit 53 and also to circuit 52, while the latter circuit is alsocoupled to circuit 53. The couplings M1, M2, and Ma are 0 indicated inFig. 8. Two circuit paths are provided between the plate circuitof tube32 and the grid circuit of tube 43, one including the tuned circuits 5|and 53, and the other including the tuned circuits 5|, 52, and 53.

In the arrangement according to Fig. 9, the windings 38 and 39 a'reseparate coils which are located outside of coils 31 and 40,respectively. As in Fig. 8, there are three mutual couplings, M1,,Mz,and Ma. Also as in Fig. 8,'two circuit paths are provided between tubes32 and 43, one including the tuned circuits 5| and 53, and the otherincluding all three tuned circuits. The arrangement shown in Fig. 9 hascertain advantages over that shown in Fig. 8, as will be explainedpresently. a

The operation of the transformer shown in Fig. 3 is similar in principleto the operation of the transformer shown in Fig. 1; that is, it dependsupon the generation of substantially equal 40 i and opposing voltages inwinding 40 at some particular frequency where attenuation is desired,which reduces the current in tuned circuit 53 to a minimum at thatfrequency. The operation of Fig. 3 differs in detail from that of Fig.1, be- 5 cause of the fact that in Fig. 3 the two circuit paths throughthe transformer include an unequal number of inductive couplings,whereas in r Fig. 1 the two circuit paths include an equal, number ofinductive couplings. The details may be brought out so far'as necessaryby continuing the discussion with reference to Fig. 4, which correspondsto Fig. 3 like Fig. 2 corresponds to Fig. 1

It may first be assumed that the winding arrangement is as shown in Fig.8. With this ar- 55 rangement it will readily be seen that all themutual couplings are positive, or at least the effect is the same aswhen they are, for reversing any one coil will simultaneously reversetwo couplings. Reversing any two coils produces the same condition asreversing the remaining coil, and of course is equally ineffective.Assuming all couplings to be positive, therefore, the equation for thecurrent I3 in the output circuit 5 can be written in the form E v (10)Defining Q2 and 62 as specified in the previous discussion of Fig. 2,and assuming that Q1 and Q3 are each equal to Q, Equation 10 can berewritten in the form (1 An inspection of Equation 11 indicates thatcurrent I: can never be reduced to zero. However, the value of 13 willbe a minimum when (12) Equation 12 can be rewritten in the form K K 2 v2 l(l and from the latter equation may be derived the equation for thefrequency of maximum attenuation in the form A of the operation of thecircuit, Fig. 4, from a functional standpoint. Bearing in mind that thethree coil structure of Fig. 8 is under discussion, and that the mutualcouplingsare all positive, it will be appreciated that at the desiredfrequency of attenuation currents I1 and I2, must be 180 degrees out ofphase with each other to produce the required opposing voltages in In.Since the voltage in L2 (coil 3839) lags the current I1 by degrees, therequired phase relation between I2 and I1 canbe obtained if current I2lags behind the voltage in L2 by 90 degrees at the attenuationfrequency. This theoretical condition cannot. be attained in practice,due to unavoidable resistance in circuit 52, as can beseen from Equation11. It can be approximated close enough, however, so that practicalresults can be secured. But the required lagging characteristic ofcurrent I2 can only be obtained at a frequency higherthan the resonancefrequency is, which lies in the picture transmission band, and hence theattenuation frequency I must be located above the transmission band, asstated at the outset.

Fig. 10 shows a typical selectivity curve such as may be obtained withthe three coil arrangement of Fig. 8. It will be noted that the maximumattenuation is not obtained at a frequency of 14.25 megacycles, but at asomewhat higher frequency. The attenuation frequency could be broughtcloser tothe transmission band, as indicated by Equation 14, but it hasbeen found in practice that it cannot be done without adverselyaffecting ,the required flat top characteristic of the transmissionband. The curve shown indicates about the best results that can beexpected 1- though the frequency of maximum attenuation with thearrangement under discussion.

is .not at the optimum point, the curve does have a much steeper slopeon the high frequency side. i

From the foregoing it will be seen that the practical results which canbe secured with the three coil arrangement of Fig. 8 fall somewhat shortof the results which might be expected from the theoretical discussion,although the selectivity of thecoupling system is distinctly improved.The partial failure is due to the inflexibility of the coil arrangementas regards the adjustment of the mutual couplings. With only threecoils, any change in one of the couplings changes another coupling atthe same time, as can readily be seen from Fig. 8. if we change couplingM2 by moving coil 31, we necessarily change coupling M1 at the sametime. Adjusting coupling M3 by moving coil 40 also changes coupling M1,and of course we cannot For instance, I

adjust coupling M1 by moving coil 31 or coil 40 without also changingcoupling M: or cou- P M3.

The difliculties referred to above are obviated by the four coilarrangement of Fig. 9, which is entirely flexible as regards independentadjustment of the'couplings, as will be apparent from inspection of Fig.9 and the circuit drawing, Fig. 4. To give just one example, coupling M1can be adjusted by moving coil 39, and this adjustment will be withouteffect on couplings M1 and M2, for the relative positions of the coilsinvolved in the latter couplings is not disturbed. The freedom incoupling adjustment afforded by the four coil arrangement gives greaterflexibility in the choice of the attenuation frequency, which can belocated close to the transmissionband without affecting the flat topcharacteristic which is essential for the correct reception of thepicture signals.

The four coil arrangement of Fig. 9 also has the advantage that theattenuation frequency may be located either above or below the picturetransmission band, being somewhat comparable in this respect to themodification which is shown in Figs. 1 and 2. This freedom as tolocation of.

particular coupling combination used determines Q the location of theattenuation frequency above or below the transmission band, -as will beexplained.

If all the mutual couplings are positive, the operation is similar tothe operation when using .the three coilarrangement insofar as thepurely mathematical treatment is concerned and the case is governed byEquations 10 to 14, inclusive. The attenuation frequency is higher thanthe resonance frequency f: of circuit 52 and lies, above thetransmission band, as indicated by Equation 14. v

For the reasons stated, however, considerably better results are securedwith the four coil arrangement as compared to those obtained with thethree coil arrangement. Fig. 11 is a typical selectivity curve, whichshows excellent attenuation on the high frequency side of thetransmission band, with a definite maximum at the.

and Equation 14 for the frequency of maximum attenuation changes tothe'form Equations 15 and 16 hold independent of the location of thenegative coupling. The operation of the transformer is alsofundamentally the same, although the phase relation of curdifferent inthe two cases, depending on whichv coil in circuit 52 is reversed inorder to produce the negative coupling. This will be explained Lz (coil39), currents I1 and I; must be in phase the voltage in L: with respectto 11, gives ,a H

rents I1 and I: at the attenuation frequency is at the frequency whereattenuation is desired, in order to produce opposing voltages in La.Since the voltage in L"2 lags the current 11 by degrees, the desiredphase relation between currents I1 and I: can be obtained if current I:leads the corresponding voltage by 90 degrees. This condition cannot bereached in practice, due to the resistance in circuit 52 but can beapproached near enough to produce effective attenuation at the desiredfrequency. Since a leading current is required, the attenuationfrequency must lie below the picture transmission band, as confirmed byEquation 16.

If coupling M2 is made negative by reversing L": (coil 38), couplings M1and M: being positive, currents I1 and I: must be out of phase bydegrees at the frequency of attenuation. The reversal of L": produces aphase shift of 180 degrees, which when added to the phase shift of phaseshift of 270 degrees for current I: with respect to current 11. Thisphase shift will be reduced to 180 degrees at the frequency ofattenuation if current I2 leads the voltage by 90 degrees. As in theprevious case, the required theoretical condition can only beapproximated, but near enough for practical results.

Fig. 12 shows a typical selectivity curve for the transformer Fig. 3,using the four coil arrangement of Fig. 9.and a negative coupling. Thecurve shows a considerable attenuation on the low frequency side, with afrequency of maximum attenuation at 8.25 megacycles, where thetransmission is down about 35 decibels. The curve of Fig. 12 isgenerally similar to the curve of Fig. 13. The results secured are notas good, but under certain conditions they will be entirelysatisfactory.

Referring now to Fig. 5, the further modification shown therein may bedescribed briefly. As in the previous modifications, the two vacuumtubes 62 and 83 may be located at adjacent intermediate frequency stagesin a television receiver. The plate circuit of tube 62 is connected tothe grid circuit of tube 83 by means of a transformer which comprisesthe four tuned circuits "ii, l2, l3, and I4. The tuned circuit H in-'cludes the variable condenser 63, a fixed condenser 64, and theinductance B'l. Tuned circuit 12 includes the variable condenser 65, thevariable resistance 80, the inductance 66, and the fixed condenser 64.Tunedcircuit' "ll includes the variable condenser .15, the inductance69, the fixed condenser 16, and the'variableresistance 8|. Tunedcircuit-13 includes the variable condenser H, the fixed condenser I6,and the inductance 68. The condenser 64 is shunted by a resistance 19 ofabout 1000 ohms to afford a direct current path for plate current." 0nthe output side the condenser 11 is shunted by the load resistor 18. Theinductances 66, 61,158, and 69 are the windings of the transformer andcomprise four coils the physical arrangement of which may be explainedwith reference to Fig. 9. Coils 68 and 68 constitute a pair of coils andmay occupy positions corresponding to the positions'of coils 3| and 31,Fig. 9. Coils 66 and 68 accordingly are inductively related. Coils s1and "constitute a second pair of inductively related coils and may issuflicient to substantially "eliminate inductive coupling between pairs;that is, there should be substantially no coupling between the twoinside coils. If necessary to attain this object, the core on which thecoils are mounted can be made somewhat longer, or the two pairs of coilscanbe mounted on separate cores. a

As regards the coupling between the several tuned circuits, it will beseen that the first tuned circuit 1|, on which high frequency signalvoltages are impressed byv the plate circuit of tube 62, is inductivelycoupled to tuned circuit ll by means of coils 61 and 69, while thelatter tuned circuit is capacitatively coupled to tuned circuit 13 bymeans of condenser I6. Also, it will be seen that tuned circuit II mcapacitativelycoupled to tuned circuit I2 by means of condenser 64,while the latter tuned circuit is inductively coupled by means of coils66 and 68 to tuned circuit 13. Thus, as in the previous modifications,two circuit paths are provided between the input and output sides of thetransformer, one including the-tuned circuits 1|, I2, and 13, and theother including the tuned circuits ll, I4, and I3. The arrangement issimilar to the arrangement of Collecting the reactive, terms of Equation19, we

derive the equation C4M1R4 52Q2 czMiRfifi (21) From Equations 20 and 21we have which is the equation for simultaneous balance. From Equation 22may be derived the equation for the attenuation frequency j in thefollowing 23 1J3} I f2Q4, i which is identical with EquationB. It willbe seen therefore that the attenuation frequency can be located eitherbelow or above the transmission band as desired.

In adjusting the transformer to secure maximum attenuation at thedesired frequency, the variable condensers C: and 0'4 are adjusted inorder to satisfy Equation 21', that is, to produce the requisite phaserelation between currents I: and I4 at the desired attenuationfrequency, after which the resistances R2 and R4 are adjusted to satisfyEquation 20, which insures the requisite Fig. 1 in that the number ofcouplings is the I same in the two circuit paths, being difierentin thisrespect from the arrangement of Fig. 3. The arrangement of Fig. 5differs from both of the previous arrangements in the fact that acapacitative coupling is used in each circuit path.

The operation of the transformer may be discussed briefly with referenceto Fig. 6, which is the equivalent theoretical circuit diagram, and willbe understood without difficulty in view of the previous explanation.Suffice it to say that at the frequency of maximum attenuation, thecurrents I2 and I4 produce substantially equal and opposing voltages incircuit 13, which reduces the corresponding current I3 substantially tozero. In

, order to effect such cancellation, it will be clear that oneof the twomutual couplings M1 and M2 must be negative. Y

The mathematical analysis of the transformer is similar to that ofthetransformer shown in Fig.

*1. "Thus we may write as the equation for the current I; in the circuit13 IFTYI For cancellation of current 13, the determinant D must equalzero, and we may write M124 M222 C C Collecting the resistive terms ofEquation 19, we derive the equation relationship as regards themagnitudes of currents I2 and I4.

No selectivity curve is shown for the transformer of Fig. 5, but it maybe stated that the results obtained are comparable with those obtainedwith the transformer of Fig. l, insofar as attenuation at some selectedfrequency is concerned. Assuming that the transformer is adjusted forattenuation at 8.25 megacycles on the lower side of the transmissionband, reference may be made to Fig. 13 to ascertain the amount ofattenuation at this frequency. That is, the low side of the selectivitycurve for Fig. 5 is substantially the same as shown in Fig. 13. Thereis, however, no additional attenuation on the high frequency side asshown in Fig. 13, but the curve on this side is about the same as isshown in Fig. 12. Of course, the frequency of attenuation f can be movedto the upper side of the transmission band, if desired, in which casethere would be no additional attenuation on the low frequency side, thelower side of the curve being similar to the lower side of the curve inFig. 10.

The invention having been described, that which is believed to be newand for which the protection of Letters Patent is desired will bepointed out inthe appended claims.

I claim:

1. In a band pass coupling system for transmitting frequencies within agiven frequency band, an input circuit including acoil, an outputcircuit including a second coil inductively coupled tosaid first coil, athird circuit including a third coil inductively coupled to said firstand second coils, and means including said couplings for tuning saidcircuits so that the frequencies within said band are transmitted withsubstantially uniform response while at a frequency close above saidband the current in the third circuit has a lagging phase angleapproaching degrees, whereby the currents in the input circuit and saidthird circuit differ in phase by an angle approaching degrees and thevoltages induced in the output circuit approximately cancel each other.

2. In a band pass coupling system for transmitting frequencies within agiven frequency band, an input circuit including a coil, an outputcircuit. including a second coil inductively coupled to said first coil,a third circuit including two coils which are inductively'coupled tosaid first and second coils, respectively, all said couplings having thesame polarity, and means including said couplings for tuning saidcircuits so that frequencies within said band are transmitted withsubstantially uniformresponse while at a frequency close above said bandthe current in the third circuit has a lagging phase angle approaching90 degrees, whereby the currents in the input circuit and said thirdcircuit differ in phase by an angle approaching 180 de grees and thevoltages induced in the output circuit approximately cancel each other.

3. In a band pass coupling system for transmitting. frequencies within agiven frequency band, an input circuit including a coil, an'outputcircuit including a second coil inductively coupled to said first coil,a third circuit including a third coil inductively coupled to said firstcoil and a fourth coil inductively coupled to said second coil, saidlast mentioned coupling being negative and all other couplings beingpositive, and means including said couplings adjusted to cause thecurrent in said third circuit to lead its voltage by a phase angleapproaching 90 degrees at a frequency close adjacent the lower side ofsaid frequency band, whereby the currents in said input and thirdcircuits are approximately in phase at such frequency and the voltagesinduced in said output circuit approximately cancel each other.

4. In a band pass coupling system for transmitting frequencies within agiven frequency band, an input circuit including a coil, an outputcircuit including a second coil inductively coupled with said firstcoil, a third circuit including a third coil inductively coupled withsaid second coil and a, fourth coil inductively coupledwith said firstcoil, said last mentioned coupling being negative and all othercouplings being positive, said last mentioned coupling and the negativepolarity thereof tending to produce a 2'70 degree phase shift betweenthe currents in said third circuit and said input circuit, and

means for reducing said phase shift to approximately 180 degrees at afrequency close adjacent the lower side of said frequency band, wherebythe voltages induced in said outputcircuit ap-- proximately cancel eachother.

5. In a band pass coupling system for transtively coupled to said inputcircuit and capacitatively coupled to said output circuit, and tuningmeans for said circuits including said couplings so adjusted thatsubstantially uniform transmission is obtained over said frequency bandwhereas at a particular frequency outside said band voltages areproduced in said output circuit which substantially cancel each other.

6. In a transformer for a band pass coupling system, four coils arrangedin pairs and so positioned relative to each other that the coils of eachpair are inductively related whereas the coupling between coils ofdifferent pairs is substantially zero, an input circuit including a coilof one pair, an output circuit including a coil of the other pair, twolink circuits including, respectively, the other coils of said pairs,and condensers coupling said link circuits, respectively, to the outputand input circuits.

7. In a transformer for a band pass coupling system, four coils locatedin spaced parallel planes on a common axis, said coils being movablealong said axis to vary the coupling between adjacent coils, input andoutput circuits including the second and third coils, respectively, acircuit including the first and fourth coils, for transferring energyfrom the input to the output circuit in addition to that transferred bythe coupling between said second and third coils and means for tuningeach of said circuits.

8. In a transformer for a band pass coupling system, five coils locatedin spaced parallel planes on a common axis, said coils being movablealong said axis to vary the coupling between adjacentcoils, input andoutput circuits including the second and fourth coils, respectively, acircuit including the third c011 inductively coupled thereby to saidinput and output circuits, a circuit including the first and iii h coilsinductively coupled to said input and output circuits by said first ingeach of said circuits.

9. In a band pass coupling system for transmitting frequencies within agiven frequency band, a tuned ouiput circuit, second and third and fifthcoils, respectively, and means for tuncircuits inductively coupled tosaid output circuit, said couplings having unlike polarities,

means for dividing an incoming signal between said second and thirdcircuits in such manner that the voltage components in said second andthird circuits are in phase", and tuning means including said couplingsfor tuning said second and third circuits to difierent frequencieswithin said frequency band, said tuning means and other constants beingso adjusted that in the vicinity of a predetermined frequency adjacentsaid frequency band the phase angles by which the can rents in thesecond and third circuits are shifted from their respective voltages arechanging slowly and are equal at said predetermined frequency, while atfrequencies within said band said currents differ in phase, the phasedifference being so related to the magnitude of the currents thatsubstantially uniform transmission is obtained over said band.

10. In a band pass coupling system for transmitting frequencies within agiven frequency band, a tuned output circuit, second and third circuitsinductively coupled to said output circuit, said couplings being of likepolarities, means for dividing an incoming signal between said secondand third circuits in such manner that the volt age components in thetwo circuits are degrees out of phase, and tuning means including saidcouplings so adjusted that in the vicinity of a predetermined frequencyadjacent said frequency band the phase angles by which the currents inthe second and third circuits are shifted from their respective voltagesare changing slowly and are equal at said predetermined frequency.

11. In a band pass filter, an output circuit, second and third circuitscoupled to said output circuit, means for dividing incoming signalcurrents between said second and third circuits, and means for tuningsaid second and third cir- -cuits.to different frequencies within adesired fredue to the difference in phase relation of the currents inthe second and third circuits while near the extremities of the band thetransmission is mainly due to the difference in the magnitude of saidcurrents, the adjustment of said tuning means and other constants beingsuch also that at a-fre'quency close outside said band the'said currentshave the proper phase relation and mag.-

' niiude to induce equal and opposing voltages in said band at whichthey are equal in magnitude.

said last means being also effective to cause said currents to coincidein phase at a selected frequency outside said band, and means foradjusting the relative magnitudes of said currents so that at saidselected frequency the opposing voltages induced in said output circuitare equal.

13 In a band pass coupling system for-transmitting frequencies .within agiven, frequency band, an input circuit, second and third circuitsconnected to said input circuit byinductive couplings, said couplingsbeing of like polarity, an output circuit connected to said secondand'third circuits by inductive couplings, said last mentioned couplingsbeing of unlike polarity; means for tuning said input and outputcircuits and the a said second circuit toflx the limits of saidfrequency band, means for tuning said third circuit to a frequencywithin said band but different than the frequencyto which the secondcircuit is tuned, means for adjusting the impedance resistance ratios ofsaidcircuits to different values correlated with the tuning and couplingfactors,

whereby the currents in the second and third circuits are caused todiffer in phase throughout 14. A band pass couplingsystem as claimed inclaim 13, wherein the couplings of unlike polarity are associated withthe input circuit rather than the output circuit, whereby the phasedifference which exists between the currents n the in which I representsthe f is to be located below or above said band, and

' second and third circuits at any frequency is v augmented by degrees.

15. In a band pass filter system for transmitting frequencies within agiven frequency band,

first and third circuits constituting input and output circuits,respectively, second and fourth circuits disposed in parallel relationbetween said input and output circuits, inductive couplings connectingthe input circuit with said second and fourth circuits, inductivecouplings connecting the second and fourth circuits with said outputcircuit, one of said couplings being negative, and tuning means in saidcircuits so adjusted that substantially uniform. transmission is securedover said frequency band while at a frequency of maximum attenuationadjacent said band but outside thereof the currents in the second andfourth circuits induce substantially equal and opposing voltages in saidoutput circuit. the frequency of maximum attenuation being determined,by the equation 1 frequency of maximum attenuation, I: and if theresonant frequencies of the second and fourth circuits, respectively,and Q: and Q4 the impedanceresistance ratios of the second and fourthcircuits, respectively.

16. A transformer for transmitting signals over I a given frequency bandand having a non-symmetrical transmission curve showing a frequency ofmaximum attenuation only on one side of said band but either above orbelow the same, said transformer comprising an output circuit, secondand third circuits coupled to said output circuit, means for dividingincoming signals between said second and third circuits, tuning meansincluding the mutual couplings, capacity, inductance, and resistance ofsaid circuits, the second and third circuits being tuned to differentfrequencies in said band and the second or the third circuit having thehigher frequency depending on whether the frequency of maximumattenuation maximum attenuation but not at frequencies immediately aboveor below the same the currents in said second and third circuits are sorelated in phase and magnitude that substantially equal and opposingvoltages are induced in said output circuit;

RIN'ALDO DE COLA.

